Long screws sometimes serve as shafts, as in large planing
machines with travelling tool, and a linear advance of the screw
may produce rotation if sufficiently large in pitch, as in Fig. 491.
(3.) Spttr Gearirtg transmits power between parallel shafts
only. Spur wheels are the equivalent of friction discs, having
teeth provided to avoid slipping with heavy loads. The teeth
are formed partly above,and partly below the disc outline, the
latter becoming virtual only, and being then termed the pitch